During recent years, along with the gradual warming of the earth caused by emission of carbon dioxide, people compellingly need for reduction of emissions of car exhaust and carbon dioxide. There is therefore a greater need for low fuel consumption of a car. And a tire is urgently required to save its weight and to reduce its rolling resistance as much as possible.
That rolling resistance reduction is also demanded not only in a tire for a passenger car but also in a heavy duty tire for a truck, bus and the like. In particular, it is important to reduce the rolling resistance of a tire for a heavy truck that runs on a pavement at high speed.
A traditional measure for rolling resistance reduction of the tire was to improve a tread rubber. However, the tread rubber was an influential member on wear resistance and grip property. Therefore, it was at the end on a technical level to reduce the rolling resistance while keeping the wear resistance and the grip property.
This created a need to reduce the rolling resistance of not only the tread rubber but also other neglected tire members.
In a general way, a pneumatic tire for heavy load is provided in a tread portion with a belt layer of plural layers (plies) made of a belt cord such as a steel cord. And, there is a belt cushion between the tire-axis-direction end of the belt layer and a carcass.
The tire-axis-direction end of the belt layer is disposed in a shoulder portion where the tread portion and the sidewall portion intersect one another. On the edge of the belt layer while the car running, a large load subjects; and there arises large stress on a stump of a rigid belt cord.
Therefore, the belt cushion needs to have abilities of relaxing a stress, preventing a separation between the belt cord and the rubber, and reinforcing a space between the tire-axis-direction end of the belt layer and the carcass sufficiently. Lack of this reinforcement causes increasing a deformation amount of the edge of the belt layer and deteriorating notably the durability.
Patent Document 1 discloses a preferable rubber composition for a belt cushion. This rubber composition comprises rubber component, carbon black, sulfur, vulcanization accelerator and the like in a specified compound ratio so as to excel in reinforcement property and improve fuel consumption.